In a conventional distance detecting apparatus mounted on a vehicle, a distance between the vehicle and an object, such another vehicle (for example, forward of the vehicle including the conventional distance detecting apparatus), may be detected using a laser beam. The distance detecting apparatus intermittently radiates the laser beam from a laser diode to irradiate the forward vehicle and detect reflected light from the forward vehicle. The distance detecting apparatus detects the distance from the vehicle to the forward vehicle based on a difference between a light radiating time and a light receiving time.
For example, a conventional distance detecting apparatus includes an irradiation portion for irradiating the laser beam, a polygon mirror, for example, a rotatable scan mirror having a truncated hexagon pyramid shape, and a laser light receiving portion for receiving the reflected light from the forward vehicle. In such a conventional distance detecting apparatus, the laser light irradiated from the irradiation portion may be reflected by the polygon mirror and introduced into a vehicle including conventional distance detecting apparatus from a front side. However, in such an arrangement, if dirt or another irregularity is present on a sensor surface, for example, a transparent surface for passing the laser beam therethrough, the distance cannot be accurately detected.